[clang][ExprConst] Add diagnostics for invalid binary arithmetic (#118475)

... between unrelated declarations or literals.

Leaving this small (I haven't run the whole test suite locally) to get
some feedback on the wording and implementation first.

The output of the sample in
https://github.com/llvm/llvm-project/issues/117409 is now:
```console
./array.cpp:57:6: warning: expression result unused [-Wunused-value]
   57 |   am - aj.af();
      |   ~~ ^ ~~~~~~~
./array.cpp:70:8: error: call to consteval function 'L::L<bx>' is not a constant expression
   70 |   q(0, [] {
      |        ^
./array.cpp:57:6: note: arithmetic on addresses of literals has unspecified value
   57 |   am - aj.af();
      |      ^
./array.cpp:62:5: note: in call to 'al(&""[0], {&""[0]})'
   62 |     al(bp.af(), k);
      |     ^~~~~~~~~~~~~~
./array.cpp:70:8: note: in call to 'L<bx>({})'
   70 |   q(0, [] {
      |        ^~~~
   71 |     struct bx {
      |     ~~~~~~~~~~~
   72 |       constexpr operator ab<g<l<decltype(""[0])>::e>::e>() { return t(""); }
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   73 |     };
      |     ~~
   74 |     return bx();
      |     ~~~~~~~~~~~~
   75 |   }());
      |   ~~~
```

The output for 
```c++
int a, b;
constexpr int n = &b - &a
```

is now:
```console
./array.cpp:80:15: error: constexpr variable 'n' must be initialized by a constant expression
   80 | constexpr int n = &b - &a;
      |               ^   ~~~~~~~
./array.cpp:80:22: note: arithmetic involving '&b' and '&a' has unspecified value
   80 | constexpr int n = &b - &a;
      |                      ^
1 error generated.

```